What do you guys think is a good rating to start grinding openings?

#18
Not by trial and error but by thinking over the board.
Capablanca lost only 48 games in his whole life.
Here is a famous game of his: his opponent played the novelty 8...d5 that he had studied secretly for 10 years. Capablanca had never seen it before, accepted the pawn, defended, and won.
https://www.chessgames.com/perl/chessgame?gid=1095025 

This is excellent advice if you are one of the greatest talents in chess history--AND you can get into a time machine and go back to 1911, when chess theory was in its infancy. Otherwise, it's garbage. There are no International Masters or FIDE Masters today who don't know a tremendous amount of opening theory, even in lines they don't play for either side. 

It's kind of the same kind of question as " What do you guys think is a good rating to start grinding tactics ?"

If we do ranking against AI in chess program, at any level if you want to progress.  

If you are noob in tactics you will be blocked due to defeat  in connection with tactical errors, if you are noob in opening you will play losing positions or garbage and you will be blocked.

Now if we do ranking against human, not so different I think.

In fact you will "work" on your weaknesses. When you lose a game if the reason for you is because of the opening you will "work" opening, if the reason for you is because of the tactics you will "work" tactics etc.

Even at the 2000 level.

If I was your tournament opponent, I spent 10 seconds on an an opening database to see that you are losing again d4 at a 43% win rate. 

I could even break it out further.

What’s causing you to lose so many games?

#22
"There are no International Masters or FIDE Masters today who don't know a tremendous amount of opening theory, even in lines they don't play for either side."
++ I know several IM and FM who play without opening theory. At GM level it is different.

#24
"What’s causing you to lose so many games?"
++ Playing too fast a time control.

I suppose you'll reference some dead person again (Capablanca, Sveshnikov, etc).

Even so, in modern times they would be the exception that proves the rule.

#27
Capablanca played without opening theory; Sveshnikov was the opposite, one of the greatest theoreticians with opening knowledge until the endgame.
Likewise there are living and active FM and IM who play without opening theory and there are also living and active FM and IM booked up as if they were World Championship Candidates.
Speaking of the World Championship Candidates': Caruana-Duda followed 24 moves of theory, Duda-Rapport and Duda-Radjabov only 7 moves.

IIRC, Duda is also the only player to have not won a single game yet

But sure, I realize some players have studied the opening a lot, and others have not done much.

I think the most sensible advice is to be a well rounded player. It doesn't make sense to ignore openings just like it doesn't make sense to prepare as if you're a world champion.

#29
It is a matter of choice. If you plan to play the Najdorf, then you cannot do without loads of theory. The FM and IM I know who play without opening theory chose early deviations. Capablanca just played sound solid moves and did neither seek early deviations nor tricky forcing lines though he did not avoid them if his opponent chose to cf. the Marshall game.

Nice fame war aside, let's return to the original question: "grinding openings" in the title bothers me quite a bit. I don't exactly know what the opening poster means by that, but it is unlikely to be anything remotely sensible. If you mean that you sit down and start memorizing computer lines, that is a complete waste of time, and that is not how grandmasters do it at all.

Then why don’t you switch to 15/10 from 10/0.

Preach what you teach. It’s hypocritical.

"Speaking of the World Championship Candidates': Caruana-Duda followed 24 moves of theory, Duda-Rapport and Duda-Radjabov only 7 moves" 

But only a fool would believe that Duda and Radjabov didn't know tons of theory. They chose to deviate into lesser known lines, but lines they knew very well

#34
15|10 is much better indeed, but
I try to reach Legend league.
It is difficult to get paired 15|10 at my level.

#35
All grandmasters know theory nowadays.
"They chose to deviate into lesser known lines"
That is the point: one of them chose to deviate early.

"That is the point: one of them chose to deviate early."

There's an enormous difference between not knowing "theory" and knowing all the theory and choosing to deviate. 

I'm of the opinion that you should try to learn whatever you can about the game.

Knowledge is power.

The key is to be realistic with your studies, and to recognize when you're trying too hard (or not trying hard enough).

If it's too confusing or too hard - you probably need to ease off.

On the other hand, if it's too easy and you don't feel challenged at all - you're probably not working hard enough.

Always work to improve your knowledge ... but don't burn yourself out, in the process.

Playing in arenas gives you bonus trophies... makes it easier.

As for Capablanca, unsurprisingly he didn't recommend ignoring the opening

"... The game might be divided into three parts, i.e.:- 1. The opening. 2. The middle-game. 3. The end-game. There is one thing you must strive for, to be equally efficient in the three parts. Whether you are a strong or a weak player, you should try to be of equal strength in the three parts. ..." - from Capablanca's book, My Chess Career

To this point, Frank Marshall spent about a decade working on what we now call the Marshall Attack, specifically to play it against Capablanca.  Capablanca refuted it over the board without ever having looked at it.

The problem with the question "when should I study openings" is that it is almost always asked by players well below the 2000-rating mark (usually by players 1200 and lower).  They think their opponents are booked up.  In reality, most 1200s have no clue what they are doing in the opening, but that isn't what is causing them to lose games.  In fact, nearly 100% of the time, the reason those players lose games is due to tactical weaknesses.

That is not to say that those players shouldn't spend any time working on their openings, but that they should do it a specific way.  Namely, as part of studying whole games.  The method I recommend is to find players you enjoy watching and play through their games.  You will pick up their opening ideas (and you can be pretty sure that if they are in the top 100, they are not blundering material in the opening) while learning the middlegame plans and endgame technique.  Additionally, when you play a game and analyze it, look up where the game left known book lines and/or where you deviated from the lines you had studied (no one can remember everything!)  Make mental notes "okay, so he played here and I was supposed to play X, but I messed up and played Y ... why does X work and Y not work?"  When you understand why a move cannot be played, you won't make the same mistake again.

There was a recent Perpetual Chess episode where GM Eugene Perylshteyn (sp) was explaining that he has met quite a few GMs who had very little knowledge of opening theory.  So it is quite apparent that there is no need to study opening theory (i.e. memorizing moves) at the club level.  Besides, suppose you spent the next 6 months memorizing all the lines in the Sicilian, and then you run into someone who plays like this:

Now all that time was spent doing something that doesn't help you at all because the opponent has you "out of book" on move 1 (granted, that approach can win you some games - as it did here when I lost on time in a winning position, but you are not getting better at chess by doing so).